Most solar cells described in the prior art can be subdivided into several categories according to their general structure.
The most common solar cells, the so-called standard cells, have their oppositely doped regions on both sides of the cell, and metallization including busbars on the illuminated side. There has long been a desire in the solar cell industry to get away with busbars on the illuminated side, as they create important shadowing losses.
In the so called back-contact solar cells, both ohmic contacts to the two oppositely doped regions of the solar cells (i.e. both series of external connecting points, also called termination points) are placed on the back or non-illuminated surface of the solar cell. A back contact solar cell thus provides termination point to the positive and negative terminals at the backside of the cell. This concept enables to reduce shadowing losses.
Back-contact solar cells fall into three main categories (for a review paper, see Prog. Photovolt: Res. Appl. 2006; 14:107-123).
Interdigitated Back-Contact (IBC) cells include those cell designs which rely upon carrier collection at a rear junction alone; they require silicium of high purity.
Emitter Wrap-Through (EWT) cells have carrier collection at both sides and rely upon current conduction from the front to the back through a large number of closely spaced through hole or via's in the cell.
Metal Wrap-Through (MWT) cells have a structure that is most closely linked to the standard cell structure. In these cells, the emitter is located on the illuminated side, the busbars moved to the rear surface, leaving only a thin front surface grid on the illuminated surface that is connected to the rear surface by extending it through a small number of metallised openings (or vias) in the wafer. WO 98/54763 discloses MWT cells.
In all types of back-contact cells, the current is collected at contact points or areas, which are then soldered to copper strips called connectors or interconnectors, at least one connector being soldered to a series of negative contact points or areas and at least one other being soldered to a series of positive contact points.
When the connector passes over an area of opposite sign, there must be placed an insulating layer between them. The practical result is that holes must be provided in the insulating layer; the holes must be made in the insulating layer and the insulating layer must be placed over the non-illuminated side in such a way that the holes face the contact points, thus allowing making the electrical connection e.g. by soldering. The making of the holes and the aligning of the insulating layer are two difficult operations. There is thus a need to find an easier connection system, avoiding the need to make holes in the insulating layer.